GH-84976: Re-introduce datetime.py and fix reprs

Without the change to the reprs, pure-python classes would have a repr
of `datetime._pydatetime.time`, etc.

Lib/datetime.py

@@ -0,0 +1,9 @@
+try:
+    from _datetime import *
+    from _datetime import __doc__
+except ImportError:
+    from _pydatetime import *
+    from _pydatetime import __doc__
+
+__all__ = ("date", "datetime", "time", "timedelta", "timezone", "tzinfo",
+           "MINYEAR", "MAXYEAR")